The present invention relates to a coding system for efficiently coding a picture signal with lesser amount of codes in a recording apparatus, data transfer apparatus and display apparatus, etc., and a decoding system thereof, and particularly to an improvement of interframe predictive coding/decoding system.
Among efficient coding methods for coding a moving picture signal with less amount codes, an interframe predictive coding method utilizing interframe correlation is widely known, which is to predict a picture signal of a frame to be coded by means of a picture signal of a frame already coded to encode predictive error only on the basis of the fact that neighbouring frames of a moving picture usually resemble each other.
However, all prior data are required for decoding a picture signal of a frame in a conventional interframe predictive coding method of recursive-type using a picture signal of a prior frame because present data is composed of all the prior data.
Accordingly, there is no great inconvenience in the conventional method when picture signals relating to a television conference are succesively transferred, whereas in the case of storage media such as a recording disc and recording tape, it is required to reset interframe prediction at a few frames in order to decode a picture signal of any frame from any portion of the media by random access and search, etc. Particularly, in the case of visual search, decoding at every few frames is required so that it is desired to reset prediction at every decoding.
An independent frame to be independently coded where the prediction is reset includes larger amount of data to be produced than a dependent frame to be coded by interframe prediction, so that data-compression ratio is reduced compared to the case where the prediction is not reset.
On the other hand, in the case of reverse reproduction in which picture signals are conversely reproduced on the time base against normal reproduction, decoding cannot be performed in the conventional prediction by means of a prior frame because a prediction signal for decoding is not obtained.
Furthemore, the prediction by means of a prior frame is one-way prediction on the time base so that prediction efficiency is deteriorated, particularly in the case of scene change in which a picture is greatly changed, proper prediction cannot be performed.
The prediction signal must be obtained by the same decoder used in a decoding system so that decoding process is required in a coding system, then the system becomes large-scale, i.e., expensive and bulky.
Furthermore, if there is deference in computation accuracy of decoding process, there is also deference generated in prediction signals and the difference is accumlated.
In order to solve the above problems, the inventor of this application has already proposed an interframe predictive encoding system (hereinafter called prior application) for improving prediction efficiency and data compression ratio by effectively utilizing independent frames in a Japanese Patent Laid-Open No. 1990-192378 which corresponds to U.S. patent application Ser. No. 07/465,747 filed on Jan. 18, 1990.
The prior application discloses that independent frames are preliminarily set by a constant number of frames for coding and frames located between two independent frames are coded by prediction using prior and posterior (old and new) frames.
Therefore, in storage media, random access can be performed by every independent frame so that visual search is made possible without loss of data and prediction adapting to variation of pictures is also made possible. This results in improvement of S/N ratio of a prediction signal and prediction efficiency.
Furthemore, coded data thus obtained are symmetrically arranged on the time base, so that reverse reproduction can be easily performed.
In the above mentioned prior application, when movement of pictures is small, preferable prediction is possible even if a reset interval N is large or interval of independent frames is large. Thus, it is desirable to fewer make N large so as to make independent frames which produce a large amount of data.
However, prediction is not performed correctly if the reset interval N is large when the movement of picture is large so that amount of data produced in dependent frames which are coded by prediction based on prior and posterior independent frames is increased. Accordingly, the reset interval N in which amount of data is minimum depends on degree of movement of pictures.
This is because this coding method is to directly predict an interframe on the basis of prior and posterior independent frames so that interframe prediction efficiency is improved as the reset interval N is decreased.
FIG. 1 shows overall variation of the amount of coded data due to intraframe and interframe coding corresponding to the reset interval N or degree of movement of pictures under the condition that picture quality in the independent and dependant frames is constant.
Amount of data per frame coded with intraframe coding is almost constant without respect to N or the degree of movement of picture, so that entire amount of data is in inverse proportion to N.
On the other hand, the amount of data per frame coded with interframe coding is increased as N is increased as is the degree of movement of picture. According to FIG. 1, it is understood that N in which the entire amount of data is minimum depends on the degree of movement of picture.